Fascinating new paper out reviewing the likely impacts that dredging may have on marine mammals. The paper by Dr. Victoria Todd and colleagues can be read here.
It is a great review of the effects both positive and negative that dredging can have on marine life. I like that is has done this as I would have gone all BAD, BAD, BAD! However there are some positives. The paper focuses mainly on marine dredging although it is important to remember it also occurs in rivers and lakes.
Start with the bad…
The paper delves into the direct and indirect impacts of dredging on marine mammals and the potential severity of those.
The direct impacts are interactions that cause physical injury to mortality. These include Noise Pollution, Turbidity and Collision.
Noise pollution occurs at lots of different levels and in areas that are already heavily trafficked. Our knowledge of the hearing range of marine mammals is still limited so we can not be sure of all the effects. However it is possible that the noise emitted could cause masking of marine mammal calls particularly cetaceans. I talk about this more in this blog post. The effects are likely to be over the short to medium term and occur as behavioural changes such as area avoidance and call masking. All this will occur concurrently with other industrial activities so it is hard to tease out the specific impacts of just dredging.
In regards to collision, the slow speed of active dredgers means that there is low risk of collisions taking place especially if managed well and avoids critical habitat and calving areas. The bigger risk would be when dredging vessels are in transit but if this is occurring in an already heavily transmitted area as tends to be the case for dredging this won’t significantly add to the risk already in place.
The seabed disturbance caused by dredging leads to increased turbidity and sediment suspension. However, many marine mammals are use to turbid environments and limited vision is not an issue even for species that do not use echolocation for prey detection.
Indirectly marine mammals can be affected by changes to their environment and also to prey availability due to dredging. The paper lists the possibilities as:
- Habitat degradation,
- Remobilization of contaminants,
- Sedimentation, and
- Increases in suspended sediment concentrations.
Entrainment is the removal of species from their environment and while no data is available on this impact on marine mammals it has been noted that it can affect their prey species by removing them and their eggs along with the sediment. Therefore along as dredging is restricted during important egg and larval stages of prey species then effects are unlikely to be detrimental.
The degradation of marine habitats is something that I would be really worried about as dredgers go about their process especially since “45 case studies worldwide found that 21 023 ha of seagrass beds were lost as a result of 26 dredging projects over a 50-year period”. Sirenians (e.g. manatees and dugongs) are entirely dependent on seagrass beds while other marine mammals utilise them for prey and they are an important habitat for prey species. However, from this review I am enlightened to the fact that due to mitigation measure the impact on seagrass habitats has been reduced and that as long as mitigation is followed and planning is conducted with care the effects are minimal.
Noise in regards to its effect on prey is very species specific. It has the potential to cause area avoidance in some fish species, and temporary threshold shifts in hearing ability which could become permanent over longer time periods. It is important to note that no study so far has focus on the specific effects of dredging noise but, this is inferred from reactions to similar noise levels. Marine mammals ability to find prey species in foraging groups could be impacts if prey become excluded from areas over time due to extensive noise disturbances.
As we know by now dredging disturbed sediment and this disturbance can cause pollutants that have accumulated over time to be remobilised into the water column. This has the potential though bioaccumulation up the food chain to prey species and then marine mammals. It is an extremely complex process and therefore hard to exactly point to dredging as the cause:
“Marine mammals accumulate high levels of contaminants irrespective of whether dredging occurs. Linking remobilization of contaminants from dredging to effects in marine mammals is challenging. Levels of toxins in blubber before, during, and after dredging are unknown, marine mammals are mobile and exposed to contaminants throughout their entire range, and effects are only likely to be discovered long after dredging ceases. Risks are highest and impacts greatest, when dredging contaminated sediments, but screening ensures they are disposed of responsibly, and not in the open ocean”
I think that quote from the authors is pretty clear on the complexities of toxin accumulation after their resuspension.
Sedimentation can reduce the health of benthic communities through disturbance and resettling. Some species can become suffocated through burial in sediment. It is, again, a very case specific issue and can impact marine mammals though their prey availability. To minimise this impact can be achieved again by the avoidance of spawning or nursery areas to minimizes large-scale losses of species, as will the reduction in dredging-related sedimentation around sensitive habitats such as oyster beds (these are important as many species depend on them for colonisation and predator protection).
Suspended sediment can like habitat degradation impact the spawning of marine mammal prey. The increased suspended sediment can also limit the vision of fish, damage gills and increase predator responses. While for filter-feeders the increased sediment means an increased energy expenditure to get food. Impacts can be minimised by planning dredging to avoid sensitives times and areas while using modelling to predict when and where sediment plumes concentrations will be highest.
Move on to the good…
After all that negativity (even though many of the impacts are minimal and short-term) lets take a look at what the authors found to be positive impacts of dredging.
The release of nutrients in sediment plumes can lead to greater diversity and an abundance of bent fauna near dredging channels. This would provided a greater amount of food availability for marine mammals. Dolphins have also been shown to favour previously dredged channels which may be due to the the structural properties of these channels aiding in prey detection and capture.
All the positive impacts are only positive up until a point after which they tend to become negative again or, the negativity is short-lived. Further to this if marine mammals are attracted to areas if high human activity this can only be a negative thing. With more marine mammals being drawn to areas of high anthropogenic activity then the risk of collisions and noise impacts increase.
Conclusions and opinions
So above you have a pretty long play-by-play of the different effects dredging can have in the marine environment. It actually seems that the effect of dredging can be minimal especially if precautions are taken to ensure that the time of year does not impact prey spawning or marine mammal breeding and mitigations measures are followed. The impact of noise
However, we can see that a lot more research needs to be done to tease out the complex impacts. Many of the effects are very species and contact specific making it difficult to make hard and fast rules about the impacts that can occur. The most likely occurrences will be avoidance behaviour, masking, short-term behavioural changes and prey availability shifts.
More likely effects include masking, avoidance and short-term changes to behaviour, and prey availability.
A very interesting paper bringing together what is currently known about dredging and its effects.
Have you read it? What did you think? Are there any other papers out there you think I should read right now?